1. Field of the Invention
The present invention relates to a process for manufacturing a tyre. In particular, the present invention relates to a process for manufacturing a tyre comprising the steps of producing and assembling the tyre structural elements on a toroidal support and precuring the inner surface of the green tyre by heating the outer surface of said toroidal support.
Furthermore, the present invention relates to a toroidal support to be used in a process for manufacturing a tyre, the outer surface of said toroidal support being heated during the precuring step of said manufacturing process.
2. Description of the Related Art
In the present description, the term “green tyre” is used to indicate the product which is obtained upon assembling tyre structural elements which include an elastomeric material in an uncured state.
Furthermore, in the present description the term “elastomeric material” is used to indicate a composition comprising at least one elastomeric polymer and at least one reinforcing filler. Said composition preferably also includes additives such as crosslinking agents and/or plasticizers.
Moreover, in the present description, the term “sinner surface” of the tyre is used to indicate the innermost surface of the tyre which, when the tyre is cured and operatively mounted on a wheel rim, comes into contact with the inflating fluid of the tyre.
The manufacturing process according to the present invention comprises the step of manufacturing a green tyre by consecutively producing and assembling together on a toroidal support the tyre structural elements, as described, for instance, in the European Patent Application No 928,680 in the name of the same Applicant.
The manufacturing process further comprises the successive steps of moulding the green tyre, so as to confer to the latter a desired tread pattern, and of curing the green tyre, so as to confer to the latter a desired geometrical conformation which is obtained by curing the elastomer material forming the tyre itself.
The moulding and curing steps of the green tyre are carried out by introducing the green tyre into a moulding cavity defined within a vulcanization mould, whose shape matches the shape of the outer surface of the tyre to be obtained, and by admitting a fluid under pressure into a diffusion interspace created between the inner surface of said green tyre and said toroidal support.
Such a manufacturing process is described, for instance, in the European Patent Application No 976,533 in the name of the same Applicant, according to which, during the pressing of the raw elastomer material against the walls of the moulding cavity, a radial expansion. is imposed to the tyre by effect of the pressurized-fluid admission. The pressurized-fluid admission is preferably carried out by means of feeding channels formed in the toroidal support and terminating at the outer surface of the latter. During the pressurized-fluid admission, the tyre is sealingly engaged at its circumferential inner edges, between the walls of the moulding cavity and the outer surface of the toroidal support, so as to delimit the diffusion interspace at the circumferential inner edges of the tyre itself. Advantageously, the heat amount which is necessary for curing the green tyre is provided to the latter through the walls of the moulding cavity and by means of a heating fluid which is introduced into the diffusion interspace. Preferably said heating fluid is the fluid under pressure used for carrying out the pressing step or is at least part of said fluid under pressure.
Therefore, in accordance with such a process, the manufacturing of a tyre is carried out in the absence of a vulcanization bladder which is commonly employed in conventional processes. The bladder is generally made of rubber and is inflated with steam and/or a high-pressure heated fluid and inserted into the green tyre, enclosed in the moulding cavity, in order to conveniently press the tyre against the walls of the moulding cavity and to provide the tyre with the desired geometric conformation as a result of the cross-linking process to which the elastomer material forming the tyre itself is submitted.
However, in processes without a vulcanization bladder as described above, the fluid under pressure directly comes into contact with the inner surface of the green tyre causing a plurality of inconveniences due to permeation of the fluid itself into the structure of the tyre not yet, vulcanized. For instance, separations between adjacent elastomeric layers or between the elastomeric material and the metallic or textile reinforcing structures may occur, or even corrosion phenomena in the metallic reinforcing materials may be unadvantageously promoted.
In order to avoid the inconveniences mentioned above, European Patent Application No 976,534, in the name of the Applicant, describes a tyre manufacturing process comprising the step of associating at least one layer of precured elastomeric material with the inner surface of the green tyre for preventing the permeation of the fluid under pressure into the inside of the green tyre itself. Said precured layer is suitable for obtaining a sufficient mechanical strength to diffusion and penetration of the fluid under pressure and, at the same time, a high fatigue strength, in particular during the moulding step of the tyre in order to avoid formation of fissures and cracks. Therefore, according to said European Patent Application the manufacturing process comprises the steps of forming at least one layer of raw elastomeric material on the outer surface of the toroidal support, so that the successive manufacturing of the green tyre is carried out on the toroidal support carrying said layer of raw elastomeric material, and of precuring said layer before introducing the green tyre into the vulcanization mould. Said precuring step is carried out by supplying heat to said layer through the toroidal support. Preferably, heating of the toroidal support is achieved thanks to the fact that the toroidal support comes from a previous moulding and vulcanization cycle or by means of infrared rays or equivalent means, such as electric resistors arranged in the toroidal support itself.
European Patent Application No 1,075,929 discloses a process for manufacturing a tyre according to which a rigid toroidal support is used as support on which the green tyre is manufactured and as moulding means for moulding the radial inner surface of the tyre. Said European Patent Application is focused to the problem of providing a toroidal support which is mechanically resistant, without compromising the easiness of assembling/disassembling the different parts of said support, good heat conductor, in order to supply heat to the green tyre for the curing thereof, and suitably light to be easily transferred along the production plant. According to said document, the toroidal support consists of a plurality of sectors, each sector being formed by two distinct portions: a principal portion, whose radially outer surface region forms the inner circumferential surface of the tyre, and a coupling portion, positioned radially internal to said principal portion and integral thereto, which is associated to a coupling device that assembles together the plurality of sectors to form the toroidal support. Furthermore, according to said document the principal part of each sector is moulded in a thermally conductive material (e.g., an aluminum alloy) and incorporates an electrical resistance for providing heat to the green tyre during the curing step. In a further embodiment, said electrical resistance can be placed in a bored housing within said principal part or can be fixed to the radially internal surface of said principal part.
Document JP 11-320,567 discloses a toroidal support on which a green tyre is manufactured, each sector of said support being contacted, in correspondence of the inner circumferential surface thereof, with a circular segment provided with resistor heaters so that a heat amount is supplied to the outer circumferential surface of the toroidal support from the inner circumferential surface thereof for curing the green tyre. The plurality of circular segments is arranged along the through hole of a cam of the toroidal support so that the circular segments, and thus the respective resistor heaters, are placed in the working position and can be retracted at the end of the curing step.
In a process for manufacturing a tyre wherein the green tyre is assembled on a toroidal support and the curing step is carried out in the absence of a vulcanization bladder, a plurality of drawbacks occurs due to the fluid under pressure contacting the inner surface of the green tyre.
The technical solution disclosed in European Patent Application No 976,534, i.e. precuring the layer of raw elastomeric material forming the inner circumferential surface of the tyre, advantageously avoids the occurring of said drawbacks. Furthermore, precuring said elastomeric layer, commonly referred to as “liner”, i.e. the elastomeric layer which is suitable for ensuring the retention of the tyre inflating fluid, is particularly advantageous also during the conformation of the tyre since the precured liner is provided with high mechanical resistance. On the contrary, in the case an uncured elastomeric layer is present, the mechanical resistance thereof is not substantially the same in each point of said layer so that, during the admission of fluid under pressure, the surface of said layer can react differently from point to point and a uniform conformation of the tyre is prevented. Said aspect is particularly true in the case the uncured elastomeric layer is obtained by assembling together a plurality of elastomeric strips as described, for instance, in European Patent Application No 928,680. Moreover, since the fluid under pressure exiting from the feeding channels provided within the toroidal support does not uniformly impact onto the liner surface, in the case the latter is uncured said fluid creates the formation of disuniformities on the inner circumferential surface of the tyre as well as of areas of different colours, facts which cause the tyre not to be accepted from an aesthetic point of view. According to European Patent Application No 976,534, the precuring of the inner circumferential surface of a green tyre is carried out by supplying heat thereto through a toroidal support which is heated in all its constitutive parts thanks to its use in a previous moulding and vulcanization cycle or by means of infrared rays or any equivalent means, such as electric resistors.
According to European Patent Application No 1,075,929 the whole body of the toroidal support is heated and the latter is used as a heat diffuser. In order to achieve such a result, the principal part of each sector of the toroidal support is provided with an electrical resistance incorporated within said principal part, which is made of thermally conductive material, so that a heat amount is supplied to the green tyre during vulcanization through the toroidal support. According to said document the electric resistance causes the heating of the principal part of each sector as well as of the anchoring part of each sector, which is integral with said principal part, so that the whole body of the toroidal support is heated.
Furthermore, document JP 11-320,567 discloses a technical solution according to which the whole body of the toroidal support is heated.
Therefore, all the technical solutions described in the prior art documents cited above are directed to a complete heating of the whole body of the toroidal support in order to transmit a heat amount to a green tyre's structural component.